Explain why increasing the temperature of a fixed mass of gas at constant pressure causes its volume to increase.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A sealed balloon is placed in a warm room. After a few hours, the balloon has noticeably expanded.
Model answer (5 marks)
Increasing the temperature increases the kinetic energy of the gas molecules.
Faster‑moving molecules hit the walls of the balloon more frequently and/or with greater force.
This increases the pressure that the gas exerts on the walls.
Because the pressure must remain constant (equal to atmospheric pressure), the balloon expands.
A larger volume means the molecules travel further between collisions with the walls, reducing collision frequency back to the original rate, so the pressure is restored to its original value.
Faster‑moving molecules hit the walls of the balloon more frequently and/or with greater force.
This increases the pressure that the gas exerts on the walls.
Because the pressure must remain constant (equal to atmospheric pressure), the balloon expands.
A larger volume means the molecules travel further between collisions with the walls, reducing collision frequency back to the original rate, so the pressure is restored to its original value.
Examiner tips
- Use the word ‘increase’ to show the causal chain, not just state facts.
- Mention that the balloon is at constant external pressure – atmospheric pressure – to justify the need for volume change.
- Show the link between kinetic energy, collision frequency/force and pressure.
- Keep the answer concise – 5 marks, so one sentence per point is sufficient.
Common mistakes
- Failing to state that the external pressure is constant, leading to an incomplete explanation.
- Confusing the role of temperature with pressure – e.g. saying the pressure increases instead of the volume.
- Using vague terms like ‘more gas’ instead of ‘more kinetic energy’ or ‘faster molecules’.
Mark scheme (5 marks)
- Increasing temperature increases the kinetic energy of the gas molecules
- Faster-moving molecules hit the walls of the balloon more frequently and/or with greater force
- This increases the pressure that the gas exerts on the walls
- Because pressure must remain constant (equal to atmospheric pressure), the balloon expands
- A larger volume means molecules travel further between collisions with the walls, reducing collision frequency back to the original rate, so pressure is restored to its original value
Key terms in this question
pressure · volume · temperature
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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